The present invention relates to an arrangement for accurately positioning a flat image and a flexible strip provided with lateral perforations.
Such an arrangement is particularly, although not exclusively, applicable to photo-etching apparatus and to apparatus for reproducing images by photographic means.
In the known art of producing integrated circuits, a wafer cut from a mono-crystal of a semiconductor material, such as silicon, is subjected to a series of operations involving doping, masking, photochemical etching and the ionic diffusion or implantation of doping materials, so as to form within the wafer a plurality of identical, regularly distributed integrated circuits. The wafer is then cut up into micro-wafers, which are generally referred to as "chips", each chip containing one basic set of integrated circuits. Each chip is provided with contact areas which are arranged in a predetermined configuration and, by means of interface conductors which are soldered to these areas, are intended to provide the electrical connection between each chip and a wiring support such as a printed circuit board.
In order to make the chips, which are of very small dimensions, easier to handle and to make them easier to mount on a wiring support, it has been proposed that the chips be attached to a flexible strip of an inextensible insulating material which, so that it can be moved along, is provided with regularly spaced lateral perforations. The strip is also provided with equidistant windows or openings to hold the chips in each of which openings terminate the free inner ends of radiating interface conductors, these ends being so arranged as to match the configuration of the contact areas on the chips. The chips are mounted on the strip by placing each of them under a respective one of the openings in the strip so that the contact areas on the chip each line up with a respective one of the inner ends of the interface conductors associated with this opening, and by then soldering the ends to the said contact areas.
Chips which have been mounted on the strip in this way may then be attached to a wiring support, such as a printed circuit board, by using a machine of the same type as that which is, inter alia, described in U.S. patent application Ser. No. 369,234 which was filed by the present Applicant on June 12, 1973. In this machine, the strip is fed forward step-by-step so as to bring each of the chips to the centre of a cutting die. The machine incorporates a soldering tool whose heating bit is adapted to engage in the die and, in the course of so engaging, to cut through the interface conductors very close to the edges of the chip. Having been detached from the strip in this way, the chip in then carried along by the soldering tool to be applied to the wiring support and soldered to it.
Given that in this machine the lateral perforations in the strip are used not only to move it along step-by-step but also to locate the chip accurately in relation to the cutting die and the soldering tool, it is important that the chips, and thus the radiating interface conductors should be very accurately positioned in relation to the lateral perforations.
The radiating interface conductors on the strip are preferably produced in the form of flat conductors which are obtained by photo-etching a thin sheet or foil conductive material which is attached to one of the faces of the strip. To this end, the foil, having been laminated to the said face of the strip, is covered with a layer of photo-sensitive lacquer. This layer is then exposed photo-graphically by a light source through a mask or transparency which bears a design corresponding to the configuration of the conductors which are to be formed. After this, the photo-sensitive lacquer is developed and the areas of the lacquer which have been exposed are removed so as to allow the areas of metal which are uncovered to be etched.
To expose the photo-sensitive lacquer through the mask which bears the design to be reproduced, apparatus has been proposed in which the metallised strip, after having been coated with its layer of lacquer, is moved along continuously, and an endless belt carrying the design to be reproduced is moved at the same speed as the strip so as to be superposed on the photo-sensitive lacquer for a predetermined distance, exposure taking place for a predetermined length of time while the endless belt and the layer of lacquer are superimposed. In this apparatus, the strip and the endless belt are, when superimposed, held in contact with one another by being gripped between toothed pinions and elastic pressure rollers. Such an arrangement prevents the strip and the belt from slipping on one another during exposure but it makes it necessary to provide lateral perforations in the endless belt which need to match exactly those in the strip. Furthermore, where the radiating conductors to be produced are of the same configuration for the whole length of the strip, it is necessary for the endless belt to be provided with designs which are absolutely identical to one another, which designs also need to be correctly positioned in relation to the perforations in the belt. The result is that not only is it a particularly costly matter to produce such an endless belt, because of the very close tolerances which must be observed, but also when the belt is driven by means of toothed pinions it is not possible to ensure that the belt and the strip are superimposed with extreme accuracy, since it is possible that the mis-alignment between perforations in the strip and the perforations in the belt which is superimposed on it may be as much as two-tenths of millimeter. The same error may then be repeated in the position of the radiating interface conductors which are produced in each of the openings in the strip, relative to the perforations in the strip. Furthermore, when it is necessary to change the endless belt in order to form on the strip radiating interface conductors whose configuration differs from that of the interface conductors formed previously, it proves a particularly awkward operation to remove the belt being used and replace it with another belt, because of the risks of damage to the perforations when they are disengaged from, or engaged onto, the teeth of the pinions.
The present invention overcomes these drawbacks and proposes a positioning arrangement which allows a flat image, such as a mask or a transparency, to be positioned with very great accuracy on a flexible, laterally perforated strip. This arrangement proves even more advantageous in that it provides very easy access to the flat image and, when used in exposing equipment designed to form radiating interface conductors on a flexible strip intended to carry chips, it makes it possible to produce on the strip a series of absolutely identical images which are perfectly positioned in relation to the lateral perforations in the strip.